U.S. Pat. No. 6,959,599 (2005) to Feldstein et al. discloses a storage tank level detector based on heating and then measuring the resistance drop of a vertical string of resistive elements.
The theory is that the rate of heat transfer is different between a mass of flowable material and the void volume above it such that for any container with a modest heat conducting capability, the container will experience a temperature gradient which is most pronounced at the interface of the contents with the void volume above the contents, and of course below that interface. That is to say, the rate of heat transfer through the wall of a container will be greater where there is a mass of flowable material located in the container than where there is a void volume above the flowable material. In other words, the rate of heat transfer through the container wall changes most abruptly at the level of the interface, and below. Thus, with the use of a thermochromatic material, a vivid color change occurring at the interface and below, will permit an observer to obtain a direct reading of the level of the flowable material within a container by discerning where the interface is located.
RAIT U.S. patent application Ser. No. 10/077,971 filed Feb. 20, 2002, for “External Liquid Level Gauge,” teaches an external liquid gauge which is adapted to be affixed vertically to the outside wall of a container. The external liquid level gauge as taught therein is in the form of an elongated strip and it comprises a layer of base material and a layer of thermochromatic materials. Furthermore, the thermochromatic layer comprises a light absorbing background and at least two regions of thermochromatic materials which are arranged upon the light absorbing background. The regions of at least two thermochromatic materials are disposed in arrays thereof and are arranged entirely along the length of the external liquid level gauge. Moreover, each of the thermochromatic materials responds chromatically within a different operating temperature range.
Accordingly, both for Feldstein and the present invention, it is desirable to provide a level detector for storage tanks for fluids that can be remotely operated, or at least that can function and provide data indicative of the level of fluid storage in a storage tank without on-site human intervention. Accordingly, any level indicator which relies on a visual indication is not at all useful. Moreover, it is the intent and purpose of the present invention to provide level detectors for storage tanks and the like which are external, and therefore do not rely on float and valve assemblies and the like, and which can therefore also be applied to a wide variety of storage tank structures.
The present invention is intended to function so as to provide an approximation of the fluid level within a storage tank. As will be seen, particularly when remote storage tanks are considered, it is unimportant to be exact, provided that an approximation to within at least a few percent of the actual fluid level within the storage tank can be arrived at. Feldstein discovered that it is quite possible to take advantage of the theory of the rate of heat transfer being different between a fluid such as a liquid, and the void volume above it, for any container which has at least a modest heat conducting capability, where such theory may be exploited remotely as a consequence of the use of elements or material which have high temperature coefficients. Feldstein determined that by appropriate spacing of heating elements vertically along the wall of a storage tank, and by applying appropriate sampling techniques to determine the difference between the rate of heat loss by conduction from various previously heated elements arranged vertically along a storage tank wall, a quite reasonable approximation of the fluid level within the storage tank can be determined.
All of this is possible because elements and materials exist that do, indeed, have appropriate high temperature coefficients; and because remote control of sampling and data communication is easily achievable.
For example, a remote location might, indeed, be connected at least by wire or wireless means into a network, a specific URL, wireless radio identity, mobile or cellular telephone number, or other electronic identity, so that it may be polled from time to time. Such polling would instruct that a level detection procedure should proceed alternatively, or as well, any remote location can be set up and programmed so that it will, on its own, periodically “wake up” and perform a level detection procedure as described hereafter.
By the provision of battery operated electronic and electrical apparatus, the present inventors have been able to provide a level detector for storage tanks for fluids that is remotely located, and which may function periodically or on demand, requiring visits to the remote location only when it is necessary to refill or empty the storage tank. Typically, the battery life of batteries that are on site at the remote location is designed and expected to be much greater than the anticipated interval between refilling visits, but nonetheless the batteries can be exchanged for new ones each or every few refilling visits since the cost of replenishing a battery is minuscule when compared to the cost of refilling the storage tank.
One problem with Feldstein's invention is that it does use considerable electricity to charge the resistors. Also both a charging and a measuring system is needed.
The present invention only uses passive tiny temperature sensors and a microprocessor at the tank to accomplish an accurate level detecting system. The delta temperatures between vertically spaced temperature sensors provide raw data that a microprocessor can use to calculate the interface between a void and oil as well as an interface between oil and the water at the bottom of the tank. These level interfaces can be viewed locally on a display and/or relayed remotely.
A new and non-obvious electronic level detector system is simpler and less expensive than any known system.